CuI /TEMPO (TEMPO=2,2,6,6-tetramethylpiperidinyloxyl) catalyst systems are versatile catalysts for aerobic alcohol oxidation reactions to selectively yield aldehydes. However, several aspects of the mechanism are yet unresolved, mainly because of the lack of identification of any reactive intermediates. Herein, we report the synthesis and characterization of a dinuclear [L12 Cu2 ]2+ complex 1, which in presence of TEMPO can couple the catalytic 4 H+ /4 e- reduction of O2 to water to the oxidation of benzylic and aliphatic alcohols. The mechanisms of the O2 -reduction and alcohol oxidation reactions have been clarified by the spectroscopic detection of the reactive intermediates in the gas and condensed phases, as well as by kinetic studies on each step in the catalytic cycles. Bis(μ-oxo)dicopper(III) (2) and bis(μ-hydroxo)dicopper(II) species 3 are shown as viable reactants in oxidation catalysis. The present study provides deep mechanistic insight into the aerobic oxidation of alcohols that should serve as a valuable foundation for ongoing efforts dedicated towards the understanding of transition-metal catalysts involving redox-active organic cocatalysts.

Institut für Chemie Fakultät 2 Technische Universität Berlin Straße des 17 Juni 135 10623 Berlin Germany

Institut für Chemie Humboldt Universität zu Berlin Brook Taylor Straße 2 12489 Berlin Germany

Institut für Physik Freie Universität Berlin Arnimallee 14 14195 Berlin Germany

Institute of Organic Chemistry and Biochemistry Czech Academy of Sciences Flemingovo náměstí 2 16610 Prague Czech Republic

Radboud University Heyendaalseweg 135 6525 AJ Nijmegen Netherlands

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The formation of hydrogen peroxide was excluded as its presence could neither be probed by iodometric titration nor by reaction with titanium oxysulfate. H2O2 is also found to be stable against disproportionation under catalytic turnover conditions (see Figure S20).